Curing of coatings through ultraviolet (UV) radiation requires efficient methods of initiating the chemical reaction responsible for the curing process. Curing of polymeric material through generation of radical species upon irradiation with UV light is widely used to produce coatings for medical devices. The paint and lacquer industry also makes use of UV-initiated curing of acrylates, where photoinitiators in many cases are employed. These two examples illustrate the diversity of UV curable coatings.
In a UV curing process, a photoinitiator moiety (low molecular weight or polymer-bound) absorbs UV light and undergoes transition to an excited state, which undergoes further processes which result in the formation of free radicals. This stage is known as initiation.
A polymer photocrosslinking process starts out with long linear polymer chains, and the initiation stage proceeds as described above. Through hydrogen abstraction, the free radicals can be transferred from the photoinitiator to the existing polymer backbone. Hereby forming new carbon-carbon bonds via radical recombination between the polymer chains providing a cross-linking of the before linear polymer chain. Such photoinitiators can be either of low molecular weight or bound in a polymer backbone.
One advantage of the later method is that a linear polymer has considerably different properties than the same type of polymer being cross-linked. The linear polymer may for example be soluble and can then be used in different production processes; it may be e.g. applied on medical devices by spraying or dip coating. The photocrosslinking process may then be initiated afterwards, cross-linking the polymer attaching it to the surface it is applied upon. It will neither dissolve nor melt.
Alternatively, the free radicals formed in the initiation stage may react with unsaturated monomers. This is then called a radical propagation stage. As the unsaturated moieties are transformed to new carbon-carbon bonds, the molecular weight of the radical grows and a new polymer chain is formed, i.e. the polymer is formed from unsaturated monomers and is cross-linked in the same process.
Until recently, polymers designed for use in coatings have relied on photoinitiators with relatively low molecular weight to initiate the cross-linking. In addition, the polymerization reactions for preparing the initial linear polymer often comprise co-reagents and catalysts of the polymerization process which also have relatively low molecular weight. Low molecular weight substances, and their by-products in the polymerization reaction, are generally difficult to remove from the resultant cross-linked polymer, but instead remain within the polymer matrix and diffuse slowly to the surface of the polymer during its lifetime. Over time, low molecular weight substances therefore leach from the polymer into the surrounding environment.
This presents particular problems in the polymers used in the medical field, as patient safety considerations limit the amount and type of substance which can leach from a given polymer. This is especially relevant if the polymer is to be used as a coating or adhesive which is designed to be in contact with the inside or outside of the patient's body. Notably, certain low molecular weight co-reagents and catalysts of polyurethane polymerization are toxic to plants and animals (e.g. dibutyltin dilaurate (DBTDL) or 1,4-diazabicyclo[2.2.2]octane (DABCO)).
Higher molecular weight photoinitiators, in particular polymeric photoinitiators, have comparably higher intrinsic viscosities which most likely result in longer diffusion times through a matrix. Migration of the UV active substances to the surface is therefore diminished when polymeric photoinitiators are used as opposed to lower molecular weight photoinitiators. Scarce literature within the field of polymeric photoinitiators suggests that development of such polymers could lead to novel applications and present solutions for existing needs, such as providing a material with negligent migration of substances to the surface/patient.
The majority of commercial polymeric initiators are based on a linear polymer backbone structures where a photoinitiator species is attached by a linking group to one (WO 96/33156) or both (U.S. Pat. No. 4,602,097) ends of a polymeric chain. While this type of structure provides a cost effective route to production of non-migratable photoinitiators, the linear structures tend to give rise to viscous oils and resinous materials. More problematic, the active photoinitiator weight fraction of the molecule is significantly reduced compared to the parent monomer and therefore a reduction in photoactivity by 50% or more is typically observed.
Polymeric photoinitiators based on a polyurethane main chain have been reported by Wei et al. (Macromolecules 2009, 42, 5486-5491). However, all materials prepared are linear polymeric structures with initiator species within the chain itself. While synthetically available, ‘in-chain’ polymeric photoinitiators tend to suffer from intrinsically lower photoactivity compared to the photoinitiator monomers. Moreover, linear polymers with in-chain aromatic moieties are prone to give materials with higher degree of crystallinity and much lower solubility compared to other polymer architectures.
Accordingly, it is an object of the present invention to provide polymeric photoinitiators having better photoactivity, in order to efficiently substitute low weight photoinitiators, where migration from the final products are critical. Additionally, it is desirable that such polymeric photoinitiators have good processing properties in the linear polymer state, for use in e.g. coating processes.
Although polymeric photoinitiators have been the subject of much research in recent years, water-soluble polymeric photoinitiators remain elusive. Some cationic thioxanthone macrophotoinitiators were reported by Corrales et al. (3. Photochem. Photobiol. A: Chem. 169 (2005) 95-100). Polycationic benzophenones have also been reported (U.S. Pat. No. 5,714,360), with molecular weights lower than 2000 Da. Detailed photochemical studies of cationic thioxanthones have been reported in a series of papers by Catalina et al. (Eur. Polym. J. 22 (1986) 347-350; Eur. Polym. J. 22 (1986) 871-875; Eur. Polym. J. 29 (1993) 125-130). While low molecular weight cationic photoinitiators discussed above are available, their undesirable properties such as high extractability with water after the completion of the UV curing process have prevented their practical use. Moreover, due to hygroscopic character of ammonium containing low molecular weight photoinitiators, the cured coatings tend to suffer from low weathering stability and water ingress can lead to complete coating disintegration over time. This issue has been addressed in U.S. Pat. No. 4,948,819, where quaternary ammonium benzophenones covalently linked to one acrylate or methacrylate group are described which can be copolymerised with other acrylates during UV curing. Although the authors demonstrate significantly reduced water extractability, the fact that potentially highly toxic and skin-irritating moieties such as (meth)acrylates have to be employed makes them unsuitable for medical coating applications.
Polymers containing multiple charged species (cationic, anionic or betaine-type) in the polymer backbone itself or as part of polymer pendant chains have found many practical uses in the textile, cosmetic (WO94/13724, US2008/0025933), pharmaceutical industries and as ink dispersants (U.S. Pat. No. 7,964,665), however, requiring further additives if they are to be UV cured. In these areas, water soluble polymers can reduce a load on the environment and the human body caused by the usage of organic solvents in most of mainstream polymer solution preparations.
A further object of the present invention is the provision of photoinitiator monomers and polymeric photoinitiators having good solubility or dispersibility in water, which allows for formulating aqueous solutions. Hereby reducing the use of organic solvents, and hence the load on the environment and the human body caused by such usage, both during production and afterwards when final products come in contact with the human body.
The present invention provides polymer photoinitiators in which the photoinitiator moiety itself becomes an integral part of the polymer, and remains so, during and after the polymerization process. Leaching of photoinitiator and photoinitiator by-products is therefore reduced or even eliminated. Polymers likely to improve medical safety are thereby obtained.
Additionally, the present invention provides photoinitiator monomers and polymeric photoinitiators being water soluble, and thereby further facilitates an improved medical safety.